This invention relates generally to television signal sync pulse separators and particularly to sync separators capable of operating reliably with non-standard sync signals.
The standard NTSC television signal has a very precisely defined relationship between the level of the tip of the sync pulse and the level of the back porch upon which the sync pulse and color burst ride. The relationship between the sync and video is also prescribed. There are many circuits and techniques available for separating sync from a composite video signal. A commonly used one develops a DC voltage by integrating the sync pulse over the duration of the horizontal line. The DC voltage is used as a reference to a comparator which separates sync pulses from the composite video signal by comparing the composite video with the DC voltage level. This system thus "slices" the sync pulse at a level between sync tip level and back porch level, the slice level being determined by the charge and discharge characteristics of the integrating network. Slicing of the sync pulse is desirable to obtain "clean" sync, e.g., pulses that are free from noise and that are not mistakenly based upon noise impulses.
With the advent of television cable systems and video cassette recorders (VCRs), the relationship of sync to video and of the sync tip level to the back porch level, as well as the width of the sync pulses often vary due to transmission equipment that may not fully comply with NTSC broadcast standards for over-the-air transmissions. It should be noted that television signals produced in cable systems and in VCRs need not conform in all respects to NTSC standards. The result is that many television signals have sync pulses of varying widths and sync pulses that are "crushed" with respect to video portions of the signal. Recovery of such sync pulses using prior art techniques is difficult at best and impossible in many situations. In prior art systems, where the sync is integrated over a complete horizontal line, for example, crushed sync may result in a slice level such that the video and noise are detected as sync, which is unacceptable. With the invention, the slice level of the sync pulse is maintained at its predetermined percentage of sync height (sync tip minus back porch) despite changes in the width of the sync pulse or changes in relative amplitude between sync tip level and video.